The present invention pertains to a circuit arrangement to ascertain and indicate that a luminous density limit condition is exceeded, wherein a comparator compares a temperature-dependent measurement value produced by taking the logarithm of a current value from a photoelectric diode with a determinable limit value containing an adjustable temperature dependency.
It is known that semiconductor elements, e.g., photoelectric diodes, exhibit different characteristics at different temperatures.
This effect occurs in particular in the exposure meter and control circuits of photographic devices, wherein the measurement and/or the shutter control effected depends on the very slight flow of current generated in a photoelectric converter by incident light. This effect has particularly increased in significance in recent times, because photoelectric converters have become more and more sensitive, i.e., they operate at lower and lower levels of illumination, at which they also, however, deliver a smaller and smaller signal currents.
Today, the operating range of high-quality photographic cameras often extends down to exposure values approaching E.sub.v -1 at 21.degree. DIN or even farther. This corresponds to a luminous density of 0.06 cd/m.sup.2. Photographs of objects at night by the full moon are thus possible. In general, cameras which measure down to such low luminous densities are equipped with silicon photoreceptors, since CdS photoresistors can no longer be employed in these cameras as a consequence of their activity at low luminous densities.
The silicon photoelectric diodes deliver signal currents of only a few pA (picoamps) at these low luminous densities. However, it is practically impossible for the user of such high-quality cameras to decide, on the basis of brightness and temperature, whether or not a photograph is still possible. When using high-speed films, achievable exposure times can result from very low scene brightness. In such circumstances, however, currents in the pA region produced in a photoelectric diode can no longer practically be processed with certainty. There is also the danger that the user will take photographs below a luminous density limit.
A difficulty exists in indicating such a luminous density limit, however, in that the currents delivered by the photoelectric diode must be compressed due to the large dynamic range.
This compression is effected, for example, by means of a semiconductor element in a logarithmic circuit. In this case a problem arises in that the virtually temperature-independent photoelectric current is translated in the semiconductor element into a voltage which is heavily temperature-dependent.
Thus, to compensate for the temperature coefficient of a semiconductor segment, it has already been suggested, in West German Laid Open Patent Application No. 28 22 035, that the voltage of the U.sub.BE segment of a second, identical transistor be added to the voltage of the U.sub.BE space of the transistor to be compensated, and that this total voltage be applied via a resistive voltage divider in a 1:1 ratio to the base-emitter space of a third transistor of the same kind, so that the collector current represents the geometric mean of the two collector currents of the first and second transistors, and thus becomes temperature-independent.
From a technological point of view, however, such a circuit arrangement is wasteful and expensive.